Referring to the prior art shown in FIG. 1, air filters are traditionally produced with a filter element 10 in which the pleats 12 of a filtering media 13 are folded to form pockets having sidewalls that are equidistant from a central plane, such as plane CP, that is ideally perpendicular relative to an imaginary reference plane P, which is tangent to an imaginary line L extending along the exterior surface of each of the folds on one side of the pleated element 10. In other words, if the plane P is considered as being horizontal for ease of explanation only, the angle V1 between planes CP and P is about 90°, i.e., within plus or minus 5 degrees of vertical. When each pocket has parallel sidewalls as shown in FIG. 1, the angle V2 between each sidewall and reference plane P is substantially equal to angle V1.
The pleats 12 are bonded together in spaced relation and separated from each other by a suitable bonding medium, such as adhesive beads 14. The element 10 may have a rectangular shape consisting of many more pleats than the few shown in FIG. 1 by way of example. Element 10 is then sealed into a rectangular frame of the same shape with corresponding vertical sides so as to present the openings of the pleats to the incoming airflow.
The “flow resistance”, which reduces the air pressure and flow rate downstream of the filter, will increase if the angle V1 deviates from the vertical because this will cause the incoming air to “bend” in order to flow into the pockets and pass through the filter media, and thereby through the filter structure. The surface area of the filter media has a direct bearing on the CFM (cubic feet per minute) rate at which air will permeate through the filtering media while still maintaining the filtering characteristics of this media.